Derivative Financial Instruments (Details 4) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012 counterparties	Dec. 31, 2011 counterparties
Derivative Financial Instruments
Cash and securities pledged as collateral by counterparties	$ 94
Securities pledged as collateral to counterparties	38
Collateral posted under MNAs, credit-risk-contingent provisions in a liability position	(20)	(76)
Collateral posted under MNAs for contracts without credit-risk-contingent liabilities	18
Credit Derivatives
Number of counter-parties	11	12
Notional amount	10,168	12,230
Credit exposure	66	108
Exposure, net of collateral	8	48
Gross liability fair value of contracts containing credit-risk-contingent features	61	153
Gross asset fair value of contracts containing credit-risk-contingent features and subject to MNAs	(40)	(69)
Collateral posted under MNAs for contracts containing credit-risk-contingent features	(20)	(76)
Maximum amount of additional exposure for contracts with credit-risk-contingent features if all features were triggered concurrently	1	8
AA-
Credit Derivatives
Number of counter-parties		1
Notional amount		25
Credit exposure		1
Exposure, net of collateral		1
A+
Credit Derivatives
Number of counter-parties	2	4
Notional amount	1,578	3,026
Credit exposure	8	26
Exposure, net of collateral	3	5
A
Credit Derivatives
Number of counter-parties	5	3
Notional amount	4,081	5,307
Credit exposure	26	15
Exposure, net of collateral	4	1
A-
Credit Derivatives
Number of counter-parties	3	2
Notional amount	892	3,815
Credit exposure	9	25
Exposure, net of collateral	1
BBB+
Credit Derivatives
Number of counter-parties	1	2
Notional amount	3,617	57
Credit exposure	23	41
Exposure, net of collateral		$ 41